Relative Dating

• Develops a sequence of events by comparing one rock layer to another --> doesn’t specify length of time between events

• 4 principles used to determine relative dating

• 1. The principle of superposition

  2. The principle of faunal succession

  3. The principle of crosscutting relations

  4. The principle of inclusion

     
     

1. The principle of superposition

• In a sequence of undeformed rock layers, the oldest rock beds are on the bottom and higher layers are successively younger

• This principle assumes that layers were horizontal when deposited and the beds are still their relative position

2. The principle of faunal succession

• Based on observation that different rock layers contain a particular type of fossilized flora (plants) and fauna (animals) called index fossils

• These fossil forms succeed each other in a specific and predictable order that can be identified over wide distances

Index fossils!

• Certain fossils are found only in rock layers of a particular geological time = index fossils

• To be an index fossil, they must meet some requirements:

• • Must be present in rocks scattered over a wide area of the Earth’s surface

  • Must have features that clearly distinguish them from all other fossils

  • The organisms from which the fossils formed must have lived during a relatively short geological time

3. The principle of crosscutting relations

• An intrusion of igneous rock (dike) or a fault (a break in the rocks that make up Earth’s crust, along which rocks on either side have moved past each other) is younger than the rock it cuts across

4. The principle of inclusion

• Relative age can also be determined where an overlaying rock layer contains particles of rock material from the layer beneath it

• • The bottom layer was eroded and the loose material on the surface became incorporated into the newly deposited layer

  • These particles, called inclusions, indicate the rocks in the lower layer are older than those on top